Foothills Antique
Tractor and Engine Club



Did you know that your generator can deliver current for either a positive or a negative ground system? Well, it can. And you can make it work in either mode. But, if yours produces current with the wrong polarity, you might have to make costly repairs.

A generator produces current by passing conductors (the armature windings) through a magnetic field. The magnetic field is produced by electromagnets that surround the armature. Basically, the field portion of your generator consists of copper windings around iron core, often referred to as the pole shoes.

The voltage and current delivered by your generator is determined by the strength of the magnetic field and the speed at which the generator is running. But, the polarity of the current is determined by the polarity of the pole shoes, or the direction of the magnetic flux or field, which is determined by the direction of current through the field windings.

OK, so you install a generator and take great care to connect the wires properly so you won't burn out your ammeter or cutout relay. What could go wrong if the wires are hooked up right?

Plenty! When you start your engine, the generator will start delivering current. Since the generator is still isolated from the electrical system by the voltage regulator or cutout relay at this point, it will start producing current based on the polarity of the pole shoes. As soon as it begins producing current, some of that current will be directed to the field windings to strengthen the magnetic flux.

According to John Deere's Fundamentals of Service manual manual, if the generator polarity is reversed, the generator will build up voltage and close the cutout relay points. This put the generator in series with the battery, and their voltages are added together. This high voltage across the points (about twice the battery voltage) can cause high current and enough heat to weld the points together.

This damage does not happen immediately. The instant the points close, the voltage is about the same on both sides of the relay coil, so very little current flows, and spring tension reopens the points. But, generator voltage will again close the points, and the cycle will repeat at a rapid rate. Heat and arcing will finally weld the points together.

When the points weld, the battery and generator are connected at all times. The low resistance of the generator allows the battery to continue to discharge through the generator. The high current can create enough heat to burn the armature.

How do you control the polarity of the pole shoes? The polarity is determined by the direction of the last current through the field windings. Since even a very small current can polarize the shoes, never assume the generator is properly polarized. You must polarize the generator every time it is disconnected or serviced.

To polarize the generator, simply make a short jumper wire to short between the battery (b or bat) and generator (g or gen) lugs on the cutout relay or voltage regulator. Only a split-second or a spark is required, so simply tap your jumper wire onto the lugs and pull them right back off.

Reference: John Deere Fundamentals of Service (FOS): Electrical Systems, Fifth Edition. 1984. Chapter 4, Charging Systems. John Deere Service Training, Dept F, John Deere Road, Moline, IL 61265